Due to their excellent electrical characteristics and comparatively simply constructed trigger electronics, IGBT (Isolated Gate Bipolar Transistor) power semiconductor modules increasingly are being used, even in failure-critical systems. Since traction control units of track-bound vehicles (locomotives, streetcars and subway trains, etc.) are often subjected to large mechanical and thermal stresses during their often decades-long service life, the users place the highest demands on the reliability and service life of the IGBT modules used.
Currently, there is no standardized method for testing the reliability of IGBT power semiconductor modules. A group of research institutes, manufacturers and users has concerned itself with this problem, and has discussed a corresponding test method. See, Proceedings of the 20th International Symposium for Testing and Failure Analysis, Los Angeles, Nov. 13-18, 1994, pp. 319-325, incorporated herein by reference.
According to the recommendations of this group, an IGBT module should be subjected to a plurality of load changes under defined conditions, and, in particular, should be rated as defective or faulty when the collector-emitter voltage (U.sub.CEsat), measured after several thousand cycles, deviates from an initial value by more than a predetermined percentage (20% divided by the number of chips in the module). Even if this condition is relaxed and a drop off of 10% below the initial voltage is still tolerated, in long-life modules the corresponding degradation first arises after some hundreds of thousands of load changes. Since the duration of the cycle (t.sub.on +t.sub.out) is typically in the range from about 10 to 20 seconds, the test of reliability/service life requires a measurement time of several weeks or even months, which is far too long for routine control in production.
A method for testing the reliability of electronic components specified in H. Koschel, et al., Zeitraffende Zuverlassigkeitsprufungen an Transistoren; offprint from Nachrichtentechnische Zeitschrift, No. 5 (1964), incorporated by reference, rests on the assumption that an increase in the loading of a test object results in an acceleration of the failure mechanism. As long as the failure mechanism is maintained, the test time can then be shortened corresponding to what is called an acceleration curve. The individual points of the acceleration curve are obtained from a plurality of failure distribution curves, measured respectively from samples of test objects, using a mathematical model that approximately describes the expected distribution of the service life.